Search

It is well recognized that dam construction aggravates eutrophication and hypoxia in river reservoirs, but the interaction between oxygen dynamics and carbon cycling is often unclear. Here we investigated the external and internal controls on oxygen consumption and effects of hypoxia on carbon dioxide (CO2) emission in a subtropical reservoir in southeast China based on detailed field measurements during 2017 and 2018. Hypoxia lasted 4 months starting in mid‐July and expanded from the bottom to near surface water. Rainstorm hyperpycnal flow and unusual hydraulics (outflow exit 40 m from the bottom) resulted in two thermoclines and enhanced the oxygen deficit in deeper water. Microbial respiration accounted for 67.4%–96.5% of total oxygen consumption in the bottom water. The increased supply of organic matter from storm runoff and to a lesser extent primary production in summer enhanced subsequent oxygen consumption. We observed an imbalance between excess CO2 production and oxygen depletion in summer and winter which was likely associated with other processes in the hypolimnion (e.g., chemoautotrophy, anaerobic degradation of organic matter, proton buffering, and nitrification). The water‐air CO2 fluxes suggest that the surface reservoir usually served as a CO2 source, but was a sink in summer due to high primary productivity. CO2 was always oversaturated in the hypolimnion; this layer was at the depth of the dam outflow and ultimately released 68% of annual CO2 efflux to the atmosphere. This research confirmed that construction of a hydropower dam has substantially altered reservoir metabolism and regulated the CO2 emission pathway. Key Points: Storm flow and external organic matter cause two thermoclines and 4‐month long hypoxiaOxygen consumption and carbon dioxide emission are imbalanced in summer and winterBoth the reservoir and downstream outflow are a net source of CO2 to the atmosphere [ABSTRACT FROM AUTHOR]

Floating macroalgae of Sargassum horneri (S. horneri) in the East China Sea (ECS) has increased in recent years, with ocean warming being one of the driving factors. Yet their possible origins, based on a literature review, are unclear. Here, using multi-sensor high-resolution remote sensing data and numerical experiments for the period of 2015-2023, we show two possible origins of the ECS floating S. horneri, one being local near the Zhejiang coast with initiation in January-February and the other being remote (> 800 km from the first) in the Bohai Sea with initiation in June-November. While their drifting pathways are revealed in the sequential remote sensing imagery, numerical experiments suggest that S. horneri from the remote origin (Bohai Sea) can hardly meander through the strong Yangtze River frontal zone, which may serve as a "wall" to prevent trespassing of surface floating seaweed to the south of the frontal zone, where S. horneri has a local origin. PLAIN LANGUAGE SUMMARY: Sargassum horneri (S. horneri) is a brown macroalgae (seaweed) abundant in surface waters of the East China Sea (ECS), which can serve as a moving habitat, but can also cause major beaching events and environmental problems. Knowledge of its origins is important to help implement mitigation strategies and understand possible ecological impacts along its drifting pathways. Using high-resolution remote sensing images and numerical experiments, we track floating S. horneri in space and time between 2015 and 2023. Two possible origins are identified, one being far away from the ECS and the other being local, both of which are known to have benthic S. horneri. The study also reveals how S. horneri are transported from their source regions resulting in large-scale distributions previously observed in medium-resolution satellite imagery., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier B.V. All rights reserved.)

It is widely recognized that the mesoscale eddies play an important part in the biogeochemical cycle in ocean ecosystem, especially in the oligotrophic tropical zones. So here a heterogeneous cyclonic eddy in its flourishing stage was detected using remote sensing and in situ biogeochemical observation in the western South China Sea (SCS) in early September, 2007. The high-performance liquid chromatography method was used to identify the photosynthetic pigments. And the CHEMical TAXonomy (CHEMTAX) was applied to calculate the contribution of nine phytoplankton groups to the total chlorophyll a (TChl a) biomass. The deep chlorophyll a maximum layer (DCML) was raised to form a dome structure in the eddy center while there was no distinct enhancement for TChl a biomass. The integrated TChl a concentration in the upper 100 m water column was also constant from the eddy center to the surrounding water outside the eddy. However the TChl a biomass in the surface layer (at 5 m) in the eddy center was promoted 2.6-fold compared to the biomass outside the eddy (p < 0.001). Thus, the slight enhancement of TChl a biomass of euphotic zone integration within the eddy was mainly from the phytoplankton in the upper mixed zone rather than the DCML. The phytoplankton community was primarily contributed by diatoms, prasinophytes, and Synechococcus at the DCML within the eddy, while less was contributed by haptophytes&#95;8 and Prochlorococcus. The TChl a biomass for most of the phytoplankton groups increased at the surface layer in the eddy center under the effect of nutrient pumping. The doming isopycnal within the eddy supplied nutrients gently into the upper mixing layer, and there was remarkable enhancement in phytoplankton biomass at the surface layer with 10.5% TChl a biomass of water column in eddy center and 3.7% at reference stations. So the slight increasing in the water column integrated phytoplankton biomass might be attributed to the stimulated phytoplankton biomass at the surface layer.